You need the NAV interface to set the clock. All of the OBC functions (MPG, Clock, Speed Limit Chime, Trip Distance, etc) are controlled by the NAV or MID.
Guys who upgrade to a new headend unit, lose this control.
You could try it blind, but someone with NAV would have to sequence the buttons for you - a difficult task but could be done if you have another truck with NAV right next to you
